Post-Doctoral Researches

Searching for subdwarf + late-type dwarf (late-M dwarfs and browndwarfs) systems within APOGEE data

Korea-Giant Magellan Telescope Science Group - Center for Large Telescopes - Korea Astronomy and Space Sciences Institute

These sources are known as HW Vir kind of systems and believed to construct a high percentage of subdwarfs despite not proven yet. The aim of this project was to look at the APOGEE spectra of late-type stars in order to find a trace of an sdB companion. Obtaining a high-quality IR observation for such systems would let us learn the physical parameters of the low-mass companion. Despite there are many scenarios to form both single and binary subdwarf stars, binarity in these stars is not well understood. Additionally, there are theories being discussed in the favour of the idea that subdwarfs are essentially a component of binary systems. Yet, the properties of the browndwarf components are mostly unknown, this keeps the formation and evolution of sdBs as a mystery. As the project went on, it evolved in a way to focus on HW Vir system itself. When the APOGEE data quality was not satisfactory to resolve the spectra of components or to improve the current knowledge of the physical parameters, we observed the system with IGRINS in February 2016 and in April 2016. At the final stage of the project, I worked on finding the appropriate models and the most effective approach to apply the model spectra. The solution of the system may help us to improve the parameter set of the cool component and also contribute to revealing the binary nature of sdB stars and common-envelope evolution.

Exploiting the APOGEE dataset for a study on chromospherically active stars

Korea-Giant Magellan Telescope Science Group - Center for Large Telescopes - Korea Astronomy and Space Sciences Institute

The main goal of this project is defining a chromospheric activity indicator in IR region and determining candidates for a ’Chromospherically Active Cool Stars Survey’. These are not very prominent objects in IR compared to optical wavelengths, but the high S/N of APOGEE data together with a systematic study may lead to better use of IR observations of cool stars. By comparing the active and inactive dwarf M (dM) stars at the IR region, it might be possible to determine a common feature of activity, which would be regarded as an indicator in IR. At similar studies, dM stars are one of the biggest exemptions and this study is a candidate to fill this void. Once such an indicator can be defined, it may be useful to find other active cool star candidates. Since the optical activity indicators barely work for dM stars, revealing an activity indicator for the IR region is very difficult yet important and it may allow us to categorise these stars.


The Extraordinary Outcomes of Common-Envelope Evolution

Prof. Dr C. S. Jeffery & Dr Gavin Ramsay

The aim of my project is to examine binary systems containing at least one unusual hot subdwarf. These systems allow us to explore possible outcomes of common- envelope (CE) ejection in a close binary. Depending on the observational results (i.e. orbital period, mass ratio, chemical abundances, etc.) a conclusion for these exotic systems can be reached regarding an idea of the formation and evolution of He-rich subdwarf binaries, especially regarding the CE evolution. Besides a general perspective on their formation, they will also help us to go further on stellar evolution and confirm our current knowledge about the outcomes of CE evolution. The first object studied was PG 1544+488, an exceptional short-period spectroscopic binary containing two helium-rich subdwarfs. I obtained and improved orbital and atmospheric parameters for each component. We concluded that in a CE binary containing two helium cores of nearly equal mass, the entire hydrogen envelopes of both stars are ejected. This places a very strong constraint on the physics of the CE ejection mechanism. The second object was BILyn and the first goal was to determine whether the relative motion of the two components could be resolved, and subsequently to obtain the dynamical mass ratio. Radial velocities of both components were measured but it was not possible to determine a period. An emission feature at Hα is also observed, which varies within a single night’s observation. The origin of this feature is discussed. Additionally, 17 subdwarf stars, are classified according to their helium abundances. For two of these; I obtained and improved orbital and atmospheric parameters for [CW83]1419-09 and recognised a variability of radial velocity for CD−31◦4800. Exotic binaries like PG1544+488 and BILyn place strong constraints on models for binary-star evolution. Further observations are required for BI Lyn, and to discover other exotic post-common- envelope binaries.


The relation between the absolute parameters of W UMa stars and their X-ray properties

Assoc. Prof. Dr F. F.Özeren

With this project, I aimed to find the X-ray luminosities of the systems in the most recent Chromospheric Active Binaries Catalogue. The X-ray flux values are taken from the ROSAT All-Sky Survey Faint and Bright Source Catalogues. A system is accepted as the source of the X-ray if the distance between the system and the X-ray source is 90′′ and below, considering the equatorial coordinates. The main purpose was to search if there is any relation between the X-ray luminosities and absolute parameters (e.g. Teff, Lbol, logg) of CABS. Although there was no apparent relationship between LX and these parameters, I defined 330 systems as X-ray sources; updated X-ray luminosities of 214 systems and defined 116 sources for the first time.